(HealthDay News) -- The e-cigarette may not be just a "healthier alternative" to smoking for teens. New research shows that teens who vape may be more apt to use tobacco cigarettes later on.

When teens smoked an e-cigarette during one month, they were up to seven times more likely to smoke tobacco in the future, researchers found.

"Youth using e-cigarettes were consistently more likely to smoke," said study author Krysten Bold, an associate research scientist at Yale School of Medicine's department of psychiatry.

But, she also noted, "We cannot determine cause and effect from these kinds of reports. There can be many reasons kids decide to smoke."

Approximately 3 million U.S. teens currently use e-cigarettes, the researchers said.

"E-cigarettes are commonly used by youth, and have many features that are appealing to kids," Bold explained.

The study included more than 800 high schoolers from three public schools in Connecticut. The students completed surveys at three different times: 2013, 2014 and 2015.

The average age of the teens at the beginning of the study was 15. There were 428 girls and the group was nearly 90 percent white.

In 2013, nearly 9 percent of the students had used an e-cigarette in the past month, while almost 5 percent had smoked a cigarette. By 2014, those numbers were 12 percent and 5.4 percent, respectively. In 2015, 14.5 percent of teens in the study had used an e-cig during the previous month, and 8.5 percent had smoked a tobacco cigarette, the survey revealed.

Between 2013 and 2014, the use of an e-cigarette during the past month was tied to a more than seven times risk of smoking a tobacco cigarette in the future. From 2014 to 2015, using an e-cigarette boosted the risk of having a real cigarette by four times, the findings showed.

The researchers also looked at the reverse relationship -- whether kids who smoked tobacco cigarettes were more likely to try e-cigarettes in the future. But they found no statistically significant relationship.

The findings "highlight that we really need to talk to kids about e-cigarettes and their potential consequences," Bold said. She added that more research is needed into possible long-term effects.

Kenneth Warner, a professor emeritus of public health at the University of Michigan, said, "This study shows an increased probability of trying a cigarette if they've tried a vape, but I don't believe they establish a causal relationship."

Warner also pointed out that the results don't quite mesh with findings from national studies that show the use of tobacco cigarettes has gone down for the past five years, and the use of e-cigarettes dropped in youth nationally in 2016.

"We're doing incredibly well on youth smoking. It's dropped by half in five years, so something right is occurring," he said.

And, Warner noted, "Vaping may prove to be a very short-lived fad, and it seems the latest data is showing that. If the numbers continue to fall, it may end looking like much ado about nothing."

He pointed to the use of hookahs -- a water pipe device used to smoke tobacco -- as an example of this occurring in the past. Use of those devices spiked in 2014 and then quickly fell back to previous levels.

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